Elastic Stent Graft

ABSTRACT

A stent graft including (1) a stent having a wall having at least one opening, an outer surface, and an inner surface and (2) a covering of a composite material having a least one expanded fluoropolymer membrane and an elastomer is provided. The cover can be used to cover the outer and/or the inner surface of the stent. The expanded fluoropolymer membrane contains serpentine fibrils. In exemplary embodiments, the fluoropolymer is polytetrafluoroethylene. The composite material may be axially and/or circumferentially wrapped around the stent. The composite material is fold-free throughout its operating diameter range and exhibits a sharp increase in stiffness at a predetermined diameter. The stent graft can be designed to have a stop point in either a radial or axial direction. The stent graft can advantageously be implanted undersized with respect to a nominal diameter without having material infolding.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation in part of and claims priority toU.S. Non-Provisional Ser. No. 13/675,764, filed on Nov. 13, 2012 andentitled “Elastic Stent Graft”, wherein such non-provisional applicationis hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to medical implants forsupporting, maintaining, or repairing a lumen, passageway, or opening ina living body, and more particularly, to a stent having thereon acovering that includes (1) an expanded polytetrafluoroethylene (ePTFE)membrane containing serpentine fibrils and (2) an elastomer.

DEFINITIONS

As used herein, the term “serpentine fibrils” means multiple fibrilsthat curve or turn one way then another.

As used herein, the term “controlled retraction” refers to causingarticles to shorten in length in at least one direction by theapplication of heat, by wetting with a solvent, or by any other suitablemeans or combinations thereof in such a way as to inhibit folding,pleating, or wrinkling of the subsequent article visible to the nakedeye.

The term “imbibed or imbibing” as used herein is meant to describe anymeans for at least partially filling at least a portion of the pores ofa porous material such as ePTFE or the like.

The term “elastic” as used herein refers to the property of a materialto be elongated upon the application of a force and that returns to itsapproximate original dimensions upon the release of the force due to theretraction force of the material.

The term “elongation” or “elongated” as used herein is meant to denotethe increase in length in response to the application of a force.

The term “elastic” as used herein refers to the property of a materialto be elongated upon the application of a force and that returns to itsapproximate original dimensions upon the release of the force due to theretraction force of the material.

The term “increase in stiffness” as used herein refers the increase inresistance to further elongation once the stop-point is reached.

For purposes of this invention, the entire device is considered to be“wrinkle-free” if within a 1 cm length of the device, the graft portionis devoid of wrinkles and folds. It is to be noted that the terms “freeof folds”, “devoid of folds”, and “fold free” are used interchangeablyherein.

BACKGROUND OF THE INVENTION

Conventional vascular grafts have long been used for vascular repair inhumans and animals. These devices are typically flexible tubes of wovenor knitted polyethylene terephthalate or porous polytetrafluoroethylene(hereinafter PTFE). Grafts of biological origin have also been used,these being typically fixed human umbilical or bovine arteries. Suchconventional vascular grafts usually require invasive surgical methodsthat expose at least both ends of the segment of the vessel to berepaired. Frequently it is necessary to expose the entire length of thevessel segment. These types of repairs cause major trauma to the patientwith corresponding lengthy recovery periods and occasionally, evendeath.

Alternative methods have evolved which use intraluminal vascular graftsin the form of adjustable stent structural supports, tubular grafts, ora combination thereof. These devices are preferably remotely introducedinto a body cavity by the use of a catheter type of delivery system.Alternatively, they may be directly implanted by invasive surgery. Theintent of these methods is to maintain patency after an occluded vesselhas been re-opened using balloon angioplasty, laser angioplasty,atherectomy, roto-ablation, invasive surgery, or a combination of thesetreatments.

Intraluminal vascular grafts can also be used to repair aneurysmalvessels, particularly aortic arteries, by inserting an intraluminalvascular graft within the aneurysmal vessel so that the prostheticdevice withstands the blood pressure forces responsible for creating theaneurysm. In addition, intraluminal vascular grafts provide a new bloodcontacting surface within the lumen of a diseased living vessel.

If the intraluminal graft used has a thin enough wall and adequateflexibility, it may be collapsed and inserted into a body conduit at asmaller diameter and at a location remote from the intended repair site.A catheter type of delivery system is then used to move the intraluminalgraft into the repair site and then expand its diameter appropriately toconform to the inner surface of the living vessel. Various attachmentmethods including the use of adjustable stents may be used to secure theintraluminal graft at the desired location without the necessity ofinvasive surgery.

Stent grafts (or covered stents) have been used extensively whereexternal support, such as in the form of rings, spirals, or metal frameswith a multiplicity of openings, and the like are required when theapplication or patient anatomy may exert forces that either could crushor collapse the intraluminal device were it not for the presence of thesupport. A support is also required in situations where the internalpressure on the device would result in undesirable diametric growth ofthe device were it not for the presence of the support. Stent grafts cantake many forms and may be constructed of a wide variety of materialsincluding, but not limited to, stainless steel, tantalum, nitinol stentmaterials and polyurethane, ePTFE, and Dacron cover materials.

SUMMARY OF THE INVENTION

It is an object of the present invention to utilize fluoropolymermembranes that exhibit high elongation while substantially retaining thestrength properties of the fluoropolymer membrane. Such fluoropolymermembranes characteristically possess serpentine fibrils.

It is another object of the present invention to provide a stent graftthat includes (1) a stent having a wall with at least one opening, anouter surface, and an inner surface, and (2) a cover affixed to thestent. The cover may be affixed to the outer surface and/or the innersurface. Additionally, the cover includes a composite material thatincludes at least one expanded fluoropolymer membrane and an elastomer.The composite material has a stop point at which no further expansion,elongation, or both occur. That is, the composite material can beradially expanded or elongated to a point at which further expansion isinhibited by a dramatic increase in stiffness. The expandedfluoropolymer membrane includes serpentine fibrils. In one exemplaryembodiment, the fluoropolymer is polytetrafluoroethylene. Thefluoropolymer membrane may include a microstructure of substantiallyonly serpentine fibrils. In at least one embodiment, the fluoropolymermembrane may include a plurality of serpentine fibrils. The serpentinefibrils have a width of about 1.0 micron or less. The endoprostheticdevice expands and contracts radially after deployment into a vessel ina body without the cover infolding into the vessel. The cover can remainwrinkle-free when radially expanded at about 80% of the nominal diameterand greater. Further, the composite material can be free of wrinklesbefore loading and after deployment of the endoprosthetic device to anominal diameter.

It is yet another object of the present invention to provide anintraluminal stent graft that includes (1) a stent having a wall with atleast one opening, an outer surface, and an inner surface and (2) acover that includes a composite material that contains at least oneexpanded fluoropolymer membrane having serpentine fibrils and anelastomer. The serpentine fibrils have a width of about 1.0 micron orless. The cover is provided on at least one surface of the stent andcovers at least a portion of the opening. The elastomer may bepositioned on the fluoropolymer membrane and/or in all or substantiallyall of the pores of the fluoropolymer membrane. In exemplaryembodiments, the fluoropolymer is polytetrafluoroethylene. The cover canbe optionally used to cover the outside and/or the inside of the stent.In addition, the cover possesses good flexibility. Additionally, thecomposite material has a stop point at which no further expansionoccurs. That is, the composite material may be radially expanded to apoint at which further expansion is inhibited by a dramatic increase instiffness. Also, the cover can be designed to distend at a relativelylow pressure until a predetermined diameter is achieved. Upon reachingthat predetermined diameter, significantly higher pressures are requiredto further distend the stent graft. The stent graft has a migrationresistance that is similar to devices that contain barbs that penetratethe vessel.

It is also an object of the present invention to provide an intraluminalgraft that includes a tubular support member that contains a compositematerial that includes at least one expanded fluoropolymer membrane andan elastomer where the tubular support has elastic properties in theaxial direction of the support. The expanded fluoropolymer membraneincludes serpentine fibrils. The serpentine fibrils have a width ofabout 1.0 micron or less. In exemplary embodiments, the fluoropolymer ispolytetrafluoroethylene. The elastomer may be present in all orsubstantially all of the pores of the expanded fluoropolymer membrane.Additionally, the fluoropolymer membrane may include a microstructure ofsubstantially only serpentine fibrils. In one or more exemplaryembodiments, the fluoropolymer membrane includes a plurality ofserpentine fibrils. The composite material has a stop point at which nofurther expansion occurs. That is, the composite material can beradially expanded to a point at which further expansion is inhibited bya dramatic increase in stiffness.

It is a further object of the present invention to provide a method ofmanufacturing a stent graft that includes (1) positioning a coverincluding a composite material on at least one surface of a stent at anominal diameter of the stent to form a stent graft and (2) permittingthe cover to bond to the stent at substantially room temperature. Thecomposite material includes at least one expanded fluoropolymer membraneand an elastomer. The cover may be positioned on the outer surfaceand/or the inner surface of the stent. An adhesive may be applied to thestent prior to positioning the cover on the stent. In at least oneexemplary embodiment, the expanded fluoropolymer membrane includes amicrostructure of substantially only serpentine fibrils. Thefluoropolymer membrane may include a plurality of serpentine fibrils.The serpentine fibrils have a width of about 1.0 micron or less.

It is also an object of the present invention to provide an intraluminalstent graft that includes (1) a stent having a wall with at least oneopening, an outer surface, and an inner surface and (2) a coverincluding a composite material that exhibits an increase in stiffnesswhen expanded to at least about 14 mm. The cover is provided on at leastone of the outer surface and the inner surface and covers at least aportion of the at least one opening. The composite material includes atleast one expanded fluoropolymer membrane and an elastomer.Additionally, the expanded fluoropolymer may include a microstructure ofserpentine fibrils. In one exemplary embodiment, the fluoropolymermembrane may include a plurality of serpentine fibrils. The serpentinefibrils have a width of about 1.0 micron or less.

It is yet another object of the present invention to provide anintraluminal graft that includes a tubular support member including (1)a composite material exhibiting an increase in stiffness when expandedto at least about 14 mm and (2) an elastomer. The tubular support memberhas elastic properties in the axial direction of the tubular supportmember.

It is an advantage of the present invention that an endoprostheticdevice containing the composite material is free or substantially freeof folds when radially expanded to about 80% of its nominal diameter.

It is also an advantage of the present invention that a stent graft canbe manufactured at the stent nominal diameter.

It is a feature of the present invention that endoprosthetic devicesutilizing the inventive elastomeric composite material demonstrateimproved migration resistance.

It is another advantage of the present invention that the stent graftcan be implanted undersized with respect to a nominal diameter withouthaving material infolding.

It is another feature of the present invention that an elastic stentgraft is formed that has regions of compression and extension withoutmaterial folding.

It is yet another feature of the present invention that the stent graftmay have elastic properties longitudinally, radially, or bothlongitudinally and radially.

The foregoing and other objects, features, and advantages of theinvention will appear more fully hereinafter from a consideration of thedetailed description that follows. It is to be expressly understood,however, that the drawings are for illustrative purposes and are not tobe construed as defining the limits of the invention.

BRIEF DESCRIPTION OF THE FIGURES

The advantages of this invention will be apparent upon consideration ofthe following detailed disclosure of the invention, especially whentaken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic illustration of an exemplary, idealized serpentinefibril;

FIG. 2 a is a scanning electron micrograph of the surface of aninventive elastomeric composite material taken at 5000×;

FIG. 2 b is a scanning electron micrograph of the surface of theinventive elastomeric composite material of FIG. 2 a taken at 5000×after subsequent elongation;

FIG. 3 is a load versus extension curve corresponding to an axiallywrapped stent according to the present invention;

FIG. 4 is a schematic illustration of an exemplary elastic stent graftaccording to the instant invention;

FIG. 4 a is a schematic illustration depicting a portion of the elasticstent graft of FIG. 4 showing a wire coated with an adhesive and thecircumferentially applied composite material;

FIG. 5 is a scanning electron micrograph of the surface of a helicallywrapped tube with the copolymer partially removed taken at 5000×; and

FIG. 6 is a pressure vs. diameter curve corresponding to a helicallywrapped tube according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which the invention belongs. In the drawings, the thicknessof the lines, layers, and regions may be exaggerated for clarity. Likenumbers found throughout the figures denote like elements. The terms“graft” and “cover” may be used interchangeably herein. In addition, theterms “stent graft” and “covered stent” may be used interchangeablyherein to describe a stent having a cover thereon.

The present invention is directed to a stent having thereon a graft thatincludes a composite material including (1) an expandedpolytetrafluoroethylene (ePTFE) membrane containing serpentine fibrilsand (2) an elastomer. The elastomer may be positioned on the ePTFEmembrane and may also, or alternatively, be located in at least aportion of the pores of the ePTFE membrane. The elastomer may be presentin all or substantially all of the pores of the ePTFE membrane. The term“substantially all of the pores” as used herein is meant to denote thatthe elastomer is present in at least a portion of all or nearly all ofthe pores of the ePTFE membrane. In one or more exemplary embodiment,the stent includes a fluoropolymer membrane.

The stent graft and tubes of the invention may be employed in any bodyconduit or vessel, including arteries and veins. Stent covers enable theexclusion of aneurysms and inhibit the passage of body fluids across thewalls of the stent. The stent cover can be optionally used to cover theoutside and/or the inside of the stent or other support structures. Itis to be understood that a tube formed of the inventive compositematerial can be used without covering a stent.

One benefit of the instant invention is that the elastic properties inthe radial direction of a stand alone tube (which could serve as anintraluminal graft) or the graft portion of the stent graft remainsfold-free when radially contracted about 80%, or conversely, the graftportion of the stent graft is fold-free when radially expanded to about80% and greater. Indeed, depending on the manner of construction, astent graft or tube formed with the inventive composite material (cover)can expand and/or contract radially after being deployed without havingor reducing the amount of material infolding, thereby minimizingobstructions that can cause thrombosis. The absence of folds whileimplanted has significant clinical ramifications, as folds in the graftportion may serve as sites of thrombus accumulation that can ultimatelyresult in total occlusion of the device. One significant importance ofthe device being fold-free is the reduction or elimination of thrombicdepositions. For purposes of this invention, the entire device isconsidered to be “wrinkle-free” if within a 1 cm length of the device,the graft portion is devoid of wrinkles and folds when viewed by thenaked eye. It is to be noted that 1 cm length of the device should beused unless the entire length of the device is less than 1 cm. In thatinstance, the entire device should be utilized to determine if thedevice is “wrinkle-free.” Consequently, should the stent graft beimplanted undersized, no folds are present in the cover. Furthermore, ifneeded, the covered stent can be elastically expanded beyond the nominalstent diameter (La, the labeled diameter) when the cover is constructedto have a stop point greater than the nominal diameter.

In another embodiment of the invention, a tube or stent graft can beconstructed such that the elastic properties are present in thelongitudinal direction of the device. A longitudinally stretchable stentgraft prevents infolding of the graft into the lumen as discussed above.One advantage of a longitudinally stretchable stent graft is that whenthe stent graft is presented with a tortuous path, the inventive stentgraft is able to bend without kinking when placed in a body conduitarch. More specifically, the tension side of the bend can stretch andthe compression side of the bend can compress without wrinkling thecomposite material (cover). The stretching of the composite materialpermits for a smooth flow through the stent graft as little or nowrinkling occurs.

The composite material may also be used to construct circumferentially-,helically-, or axially-oriented tubes and covers, as well ascombinations thereof. As used herein, the term “axial' isinterchangeable with the term longitudinal.” When the composite materialis applied helically, elasticity is imparted in both the longitudinaland circumferential directions. It was surprisingly discovered thatthere is an absence of folds in an axially formed cover material whenthe stent graft is both compacted and expanded to about 80% of thenominal diameter. The cover can be formed prior to application to astent (i.e., a separate cover can be made in the shape of a tube fromthe composite material). Alternatively, the cover can be formed afterthe composite material has been applied to the stent, after which thecomposite material is bonded both to itself and to the stent.

Stent grafts with minimal or no material infolding are able toaccommodate the natural tapering of vessels to a greater degree thanconventionally known devices. Additionally, tubes and stent graftsformed from the inventive composite material have improved migrationresistance compared to conventional devices.

The cover of the present invention is unique in that it is fold-free orsubstantially fold-free as implanted; e.g., the cover can besubstantially fold-free when radially expanded to about 80% and up toabout 100% of the nominal diameter or greater. In addition, the cover isunique in that it exhibits a sharp increase in stiffness at apredetermined diameter. This latter property creates a “stop-point”beyond which increased balloon pressure does not further increase thecovered stent diameter. One advantage of the feature of the stop pointis that the stent graft does not itself become aneurismal. The tubes andstent grafts of the present invention can be designed to have a stoppoint in either a radial or axial direction.

An additional benefit of the present invention is that the tube or thecover portion of a stent graft can be designed to distend at arelatively low pressure until a predetermined diameter is achieved. Uponreaching that diameter, significantly higher pressures are required tofurther distend the tube or stent graft. In other words, the slope ofthe diameter versus pressure curve decreases dramatically once thepre-determined diameter is reached.

Pressure-diameter curves relating to tubes and covered stents of thepresent invention exhibit an inflection point due to the change in slope(which is directly related to the stiffness) upon reaching a diameterreferred to herein as the stop point. FIG. 6 is a pressure vs. diametercurve of an article according to the present invention, in this case atube, in which the intersection of two tangent lines depicts the stoppoint of the article. An estimate of the stop point may be determined inthe following manner. The slope of the pressure-diameter curve prior toreaching the stop point can be approximated by drawing a straight linetangent to the curve, shown as line 30 in FIG. 6. The slope of thepressure-diameter curve beyond the stop point can be approximated bydrawing a straight line tangent to the curve, shown as line 40 in FIG.6. The diameter corresponding to the intersection of the two tangentlines, depicted by reference numeral 50, is an estimation of the stoppoint for the article.

A stent graft according to the invention can advantageously beconstructed at the stent nominal (i.e., labeled) diameter, or a largerdiameter. In particular, a cover formed of the composite material can bepositioned on the outer and/or inner surface of the stent with the stentat the nominal stent diameter. In one embodiment, the composite materialis at least partially elongated upon application to the stent (e.g., bywrapping the material around the stent) at its nominal diameter, orlarger than nominal diameter. In an alternate embodiment, the compositematerial may be stretched until the composite material is at leastpartially elongated prior to or during the positioning of a tubularcover on the stent. For example, a cover having a substantially smallerdiameter than the nominal diameter of the stent can be employed to coverthe stent at least partially due to the elasticity of the compositematerial. The range within which the cover can be wrinkle-free can be atleast about 80% of the nominal diameter and greater

During construction of a tube or a cover for a stent graft, the amountof tension applied to the composite material when forming can beadjusted to adjust the elastic range (or wrinkle-free range) of thecover and the stop point. For example, if the composite material isapplied to a stent at nominal diameter with near complete elongation,the stop point will be approximately the nominal diameter, and theamount of compaction permitted while remaining wrinkle free and theamount of expansion permitted while remaining wrinkle free is at amaximum for that composite material. However, if the same compositematerial is applied to the same stent with only partial elongation, thestop point will be at a diameter greater than the nominal diameter, andthe amount of compaction and expansion permitted while remaining wrinklefree is less than the former.

The cover can be bonded to the stent at or substantially at roomtemperature. Unlike some conventional processes, no cold crushing orother similar process steps is involved. An adhesive, such as anadhesive copolymer, may be applied to the stent prior to positioning thecover on the outer surface and/or the inner surface of the stent to bondthe cover to the stent.

Another advantage of the present invention is the ability of the stentgraft to be expanded in situ to dislodge a clot or other adhesion on theinner surface. In particular, the at least partially occluded coveredstent may be expanded to a larger diameter through the use of a ballooncatheter, thereby dislodging the stenotic material. A protective measurecould be used to capture the stenotic material to prevent stroke,thrombosis, or other unwanted, deleterious side effects. Upon release ofthe pressure of the balloon, the covered stent returns at leastsubstantially to its previous diameter without wrinkling.

In at least one embodiment of the present invention, fluoropolymermembranes that exhibit high elongation while substantially retaining thestrength properties of the fluoropolymer membrane are utilized as thecover material. Such membranes characteristically possess serpentinefibrils, such as the idealized serpentine fibril exemplified in FIG. 1.As depicted generally in FIG. 1, a serpentine fibril curves or turnsgenerally one way in the direction of arrow 10 then generally anotherway in the direction of arrow 20. It is to be understood that theamplitude, frequency, and periodicity of the serpentine-like fibrils asexemplified in FIG. 1 may vary. In one embodiment, the fluoropolymermembranes are expanded fluoropolymer membranes. Non-limiting examples ofexpandable fluoropolymers include, but are not limited to, expandedPTFE, expanded modified PTFE, and expanded copolymers of PTFE. Patentshave been filed on expandable blends of PTFE, expandable modified PTFE,and expanded copolymers of PTFE, such as, for example, U.S. Pat. No.5,708,044 to Branca; U.S. Pat. No. 6,541,589 to Baillie; U.S. Pat. No.7,531,611 to Sabol et al.; U.S. patent application Ser. No. 11/906,877to Ford; and U.S. patent application Ser. No. 12/410,050 to Xu et al.

The high elongation is enabled by forming relatively straight fibrilsinto serpentine fibrils that substantially straighten upon theapplication of a force in a direction opposite to the compresseddirection. The creation of the serpentine fibrils can be achievedthrough a thermally-induced controlled retraction of the expandedpolytetrafluoroethylene (ePTFE), through wetting the article with asolvent, such as, but not limited to, isopropyl alcohol or Fluorinert®(a perfluorinated solvent commercially available from 3M, Inc., St.Paul, Minn.), or by a combination of these two techniques. Theretraction of the article does not result in visible pleating, folding,or wrinkling of the ePTFE, unlike what occurs during mechanicalcompression. The retraction also can be applied to very thin membranes,unlike known methods. During the retraction process, the fibrils notonly become serpentine in shape but also may also increase in width.

The precursor materials can be biaxially expanded ePTFE membranes. Inone embodiment, materials such as those made in accordance with thegeneral teachings of U.S. Pat. No. 7,306,729 to Bacino, et al. aresuitable precursor membranes, especially if small pore size articles aredesired. These membranes may possess a microstructure of substantiallyonly fibrils. The precursor membrane may or may not be amorphouslylocked. Additionally, the precursor membrane may be at least partiallyfilled, coated, or otherwise combined with additional materials.

The precursor membrane may be restrained in one or more directionsduring the retraction process in order to prescribe the desired amountof elongation of the final article. The amount of elongation is directlyrelated to, and is determined by, the amount of retraction.

In one embodiment, retraction can be achieved in a uniaxial tenter frameby positioning the rails at a distance less than the width of theprecursor membrane prior to the application of heat or solvent or both.When using a biaxial tenter frame, one or both of the sets of grips,pins, or other suitable attachment means can similarly be positioned ata distance less than the dimensions of the precursor membrane. It is tobe appreciated that these retraction means differ from the mechanicalcompression taught by the House and Sowinski patents noted above. Uponretraction, the expanded fluoropolymer membrane possesses serpentinefibrils. These retracted membranes characteristically possess serpentinefibrils and are substantially wrinkle free. In some exemplaryembodiments, the retracted membranes may possess a microstructure ofsubstantially only serpentine fibrils. In at least one embodiment, thefluoropolymer membranes include a plurality of serpentine fibrils. Asused herein, the phrase “plurality of serpentine fibrils” is meant todenote the presence of 2 or more, 5 or more, 10 or more, or 15 or moreserpentine fibrils in the fluoropolymer membrane within a field of viewas taught below. The serpentine fibrils have a width of about 1.0 micronor less, and in some embodiments, about 0.5 microns or less. In oneembodiment, the serpentine fibrils have a width from about 0.1 to about1.0 microns, or from about 0.1 to about 0.5 microns.

The precursor membranes described above can be imbibed with anelastomeric material prior, during, or subsequent to retraction to forma composite material. In the absence of such elastomeric materials,fluoropolymer articles having serpentine fibrils do not exhibitappreciable recovery after elongation. Suitable elastomeric materialsinclude, but are not limited to, PMVE-TFE (perfluoromethylvinylether-tetrafluoroethylene) copolymers, PAVE-TFE (perfluoro(alkyl vinylether)-tetrafluoroethylene) copolymers, silicones, polyurethanes, andthe like. It is to be noted that PMVE-TFE and PAVE-TFE arefluoroelastomers.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples illustrated belowwhich are provided for purposes of illustration only and are notintended to be all inclusive or limiting unless otherwise specified.

Testing Methods

It should be understood that although certain methods and equipment aredescribed below, any method or equipment determined suitable by one ofordinary skill in the art may be alternatively utilized.

Mass, Thickness, and Density

Membrane samples were die cut to form rectangular sections about 2.54 cmby about 15.24 cm to measure the weight (using a Mettler-Toledoanalytical balance model AG204) and thickness (using a Käfer Fz1000/30snap gauge). Using these data, density was calculated with the followingformula: ρ=m/(w*l*t), in which: ρ=density (g/cm³), m=mass (g), w=width(cm), l=length (cm), and t=thickness (cm). The average of threemeasurements was reported.

Matrix Tensile Strength (MTS) of Membranes

Tensile break load was measured using an INSTRON 122 tensile testmachine equipped with flat-faced grips and a 0.445 MI load cell. Thegauge length was about 5.08 cm and the cross-head speed was about 50.8cm/min. The sample dimensions were about 2.54 cm by about 15.24 cm. Forhighest strength measurements, the longer dimension of the sample wasoriented in the highest strength direction. For the orthogonal MTSmeasurements, the larger dimension of the sample was orientedperpendicular to the highest strength direction. Each sample was weighedusing a Mettler Toledo Scale Model AG204, then the thickness wasmeasured using the Käfer FZ1000/30 snap gauge; alternatively, anysuitable means for measuring thickness may be used. The samples werethen tested individually on the tensile tester. Three different sectionsof each sample were measured. The average of the three maximum loadspeak force) measurements was reported. The longitudinal and transversematrix tensile strengths (MTS) were calculated using the followingequation: MTS=(maximum load/cross-section area)*(bulk density ofPTFE)/(density of the porous membrane), where the bulk density of thePTFE was taken to be about 2.2 g/cm³.

Scanning Electron Microscopy

Scanning electron micrographs were created choosing magnificationssuitable for identifying fibrils. Articles that have been retracted inaccordance with the teachings of invention may require elongation in thedirection of retraction in order to identify the serpentine fibrils. Forthe purposes of identifying the number of serpentine fibrils, a field ofview of 7 microns by 7 microns of the sample is to be employed.

In addition, for the purpose of characterizing fibril width,measurements should be made for serpentine fibrils that aresubstantially separated from each other and do not band together orotherwise form series of fibrils paralleling each other within themembrane. To determine the fibril width, a line is drawn through the SEMimage to bisect it. The SEM image should be of sufficient magnificationsuch that at least 5 serpentine fibrils and not more than 20 serpentinefibrils are clearly visible within the SEM image. Starting from one edgeof the bisected image, the width of the first five consecutiveserpentine fibrils that intersect the bisecting line is measured. Themeasurements are made where the fibril intersects the bisecting line.Next, the five measurements are averaged and the average measurement isrecorded.

Removal of Elastomer from a Stent Cover

For a tube or a stent cover containing an elastomer, the elastomer canbe dissolved or degraded and subsequently rinsed away using anappropriate solvent in order to measure or examine desired properties.

For instance, the fluoroelastomer component of a tube or a stent coveras described in Examples 2-5 can be partially or substantially removedto enable SEM imaging of the ePTFE structure. The sample is submerged in95 g of Fluorinert® Electronic Liquid FC-72 (3M Inc., St. Paul, Minn.)and allowed to soak without agitation. After approximately one hour, thefluorinated solvent is poured off and replaced with 95 g of freshsolvent. This process is repeated for a total of 5 soaking cycles, thefirst 4 cycles for approximately 1 hour, and the 5th cycle forapproximately 24 hours.

To aid in the removal of elastomer, the sample can also be agitatedusing an ultrasonic cleaner (e.g. Branson 200 Ultrasonic Cleaner(Model—B200)).

Pressure vs. Diameter Test

A pressure versus diameter curve is created by radially expanding a tubeor covered stent using an inflation syringe to pressurize a ballooncatheter. The tube or covered stent is positioned over the ballooncatheter in its deflated state. The tube or covered stent diameter ismeasured at 1 ATM intervals using a laser micrometer.

For example, a 14 mm balloon diameter with a 20 mm balloon length and a0.035 inch guide wire Peripheral Dilation Catheter (Agilitrac, AbbottVascular, Santa Clara, Calif.) and a Basix™ COMPAK (Merit Medical, SouthJordan, Utah) inflation syringe were used. The handle on the inflatorwas slowly turned while the change in pressure as indicated on the dialwas observed. The balloon was then inflated to 2 atmospheres (ATM) ofpressure. The stent easily continued to expand, causing the pressure todrop. The inflation was continued until the pressure of 2 ATM wasmaintained. The stent was then inflated at 1 ATM intervals and thediameters at those pressures once pressure equilibrated were recorded.Inflation was continued until 7 atmospheres was achieved, which was 1ATM below the rated burst pressure of the balloon. The tube diameter wasrecorded at each 1 ATM interval using a laser micrometer (LaserMike™Dayton Ohio).

A similar procedure should be followed for different sizes of tubes andcovered stents. For other tube and covered stent sizes, an appropriatelysized balloon is chosen. Continue inflating until reaching 1 ATM belowthe rated burst pressure of the balloon.

Axial Elongation Test

The axial elongation test is performed by elongating a sample in atensile testing machine at a constant crosshead speed. The gage lengthof the sample and the crosshead speed are recorded. The intent of thetest is to determine the stop point of a sample, should one exist.

The load-extension curves relating to composite materials and coveredstents and tubes of the present invention exhibit an inflection pointdue to the change in slope upon reaching a length referred to as thestop point. An estimate of the stop point is determined in the followingmanner. The slope of the load-extension curve prior to reaching the stoppoint can be approximated by drawing a straight line tangent to thecurve. The slope of the bad-extension curve beyond the stop point can beapproximated by drawing a straight line tangent to the curve. Anestimate of the stop point is the extension corresponding to theintersection of the two straight lines.

EXAMPLES Example 1

An elastomeric composite material was made in the following manner.

Precursor Membrane

A biaxially expanded ePTFE membrane that had not been amorphously lockedand had the following properties was obtained: thickness=0.0023 mm,density=0.958 g/cc, matrix tensile strength in the strongestdirection=433 MPa, matrix tensile strength in the direction orthogonalto the strongest direction=340 MPa, elongation at maximum load in thestrongest direction=39%, and elongation at maximum load in the directionorthogonal to the strongest direction=73%. Upon tensioning by hand, themembrane did not noticeably retract upon the release of the tension.

Retracted Membrane

A roll of precursor membrane where the length direction correspondedwith the weakest direction of the membrane was restrained in the clampsof a heated, uniaxial tenter frame and fed into the heated chamber ofthe tenter frame. The oven temperature was set to about 270° C. Therails of the tenter frame within the heated chamber were angled inwardin order to allow membrane shrinkage to about 39% of its original widthin response to the heat. The line speed was set to provide a dwell timeof about 1.5 minutes within the heated chamber.

The initial and final widths of the membrane were 1625 mm and 632 mm,respectively. The retracted membrane had the following properties:thickness=0.003 mm, density=1.36 g/cc, matrix tensile strength in thestrongest direction of the precursor membrane=158 MPa, matrix tensilestrength in the direction orthogonal to the strongest direction of theprecursor membrane=409 MPa, elongation at maximum load in strongestdirection of the precursor membrane=301%, and elongation at maximum loadin the direction orthogonal to the strongest direction of the precursormembrane=85%.

Extruded Elastomer

A copolymer comprising tetrafluoroethylene (TFE) and perfluoro(methylvinylether) (PMVE) as described in U.S. Pat. No. 7,049,380 to Chang, etal. was obtained with a PMVE/TFE ratio of 2:1. The copolymer wasextruded at about 350° C. into a thin film. The film had the followingproperties: thickness=0.025 mm and width=115 mm.

Elastomeric Composite Material

The extruded elastomer was fed onto the surface of the retractedmembrane and spooled with a 0.064 mm thick high density polyethylenerelease film. The elastomeric composite material had the followingproperties: thickness=0.033 mm and width=115 mm. The elastomericcomposite material is shown in FIG. 2 a, a SEM of the surface of themembrane opposing the extruded elastomer, taken at 5,000× magnification.A length of the elastomeric composite material was stretched by hand toabout 78% of the original length. The fibrils were seen to have aserpentine shape as indicated in FIG. 2 b, a SEM of the surface of themembrane opposing the extruded elastomer, taken at 5,000× magnification.

Example 2

A circumferentially covered stent was constructed in the followingmanner. Nitinol wire (600315-18, Fort Wayne Metals, Fort Wayne, Ind.)was obtained and formed into a stent by winding the wire onto a windingmandrel with appropriately located pins. The stent was 10 cm long and 10mm in diameter. An ePTFE-fluoropolymer elastomeric composite material asdescribed in detail above which had the following properties: filmthickness=0.006 mm elastomer thickness=0.025 mm was obtained. A liquidsolution of PAVE-TFE, as described previously, and Fluoroinert® (3MFluoroinert, 3M Specialty Chemicals Division, St. Paul, Minn.) wasobtained and combined to form a solution. The stent was removed from thewinding mandrel and dipped into the solution. The stent was air driedthen placed onto a 10 mm mandrel that had a covering of Kapton film (Lot#B4N0C38500ADD, E. I. du Pont de Nemours and Company, Wilmington, Del.).One end of the mandrel was secured inside the chuck of a drill. Theelastomeric composite material was cut to about 100 mm in width. It wasnoted that the length direction of the composite material was thedirection that possessed the elastomeric properties.

With the drill secured, the composite material was circumferentiallyapplied to the stent such that the elastic direction of the material wastangent to the axial direction of the stent. The elastomer side of thecomposite material was applied to the stent with pressure and wassecured to the stent under tension due to the tackiness of theelastomer. Tension was applied to the composite material to elongate it.The amount of tension was sufficient to exceed the retraction forcewithout disrupting the material but lower than the stop point. The drillrotated the mandrel until one composite material layer was wrapped ontothe stent. A compression wrap was applied to the covered stent in thefollowing manner. A layer of Kapton film was applied, followed by theapplication of an ePTFE film applied under tension to ensure uniformcontact between the composite material and the stent. The assembly wasplaced in an oven set to about 50° C., for about 24 hours. The stentplus films was removed from the mandrel and the outer ePTFE film andKapton films were removed, thereby resulting in a covered stent. FIG. 4is a schematic illustration of a cross-sectional view of an exemplaryelastic stent graft 100 according to the instant invention. FIG. 4 a,schematically depicts the wire 110 coated with an adhesive 120 and thecomposite material 130 circumferentially applied to the stent.

When the covered stent was crushed from 10 mm to 8 mm by pulling itthrough a funnel, no folds in the stent cover were observed.

Example 3

An axially covered stent was made as described in Example 2 with thefollowing exceptions. The composite material was applied such that thedirection of the film possessing the elastomeric properties was in theaxial direction of the stent.

The axial elongation test performed with a gage length of 82.5 mm and acrosshead speed of 60 mm/min demonstrated that the stent graft axiallyelongated and incorporated a stop point at about 30 mm. FIG. 3 is a loadvs. extension curve corresponding to the axially wrapped stent showingthe stop point 80 at about 30 mm, where tangent lines 60, 70 intersect.The stent graft recovered to approximately its original length uponremoval of the tensile force without substantial wrinkling.

Example 4

An axially wrapped tube was made in accordance with the processdescribed in Example 2 with the following exceptions. A tube wasconstructed without involving a stent. The axial elongation testdescribed above demonstrated that the tube axially elongated andincorporated a stop point. The tube recovered to approximately itsoriginal length upon removal of the tensile force.

Example 5

A helically wrapped tube was made in accordance with the processdescribed as follows. An ePTFE-fluoropolymer composite material asdescribed in Example 1 was obtained having the following properties:film thickness=0.006 mm, elastomer thickness=0.025 mm. The compositematerial was slit to about 100 mm wide. A helically wrapped tube wasmade by first wrapping a Kapton film covering ((Lot #B4N0C38500, E. I.du Pont de Nemours and Company, Wilmington, Del.)) over a 10 mmstainless steel mandrel and then wrapping one layer of the compositematerial, with the elastomer side facing outward, on top of the Kaptonfilm at an angle of approximately 45 degrees relative to thelongitudinal axis of the mandrel. Then, another layer of the compositematerial was wrapped over the first layer of composite material, butwith the elastomer layer facing inward and at an angle 90 degreesrelative to the first composite material. In this way, the helicallywrapped tube had two layers of composite material, one layer at 45degrees relative to the longitudinal axis and another layer at 135degrees relative to the longitudinal axis.

A compression wrap was applied to the outer layer in the followingmanner. A layer of Kapton film was helically wrapped over the outermostcomposite material, followed by an ePTFE film applied under tension toensure uniform contact between the composite film layers. The resultingassembly was placed in an oven set to 50° C. for approximately 24 hoursto bond the two layers of the composite material to each other. Thecoverings and helically wrapped tube were removed from the mandrel andthe coverings were removed from the tube. The resulting helicallywrapped tube was 10 cm long and 10 mm in diameter.

The axial elongation test performed with a gage length of 82.5 mm and acrosshead speed of 10 mm/min demonstrated that the tube axiallyelongated and incorporated a stop point at about 12 mm. The tuberecovered to approximately its original length upon removal of thetensile force.

FIG. 6 is a pressure vs. diameter curve corresponding to the helicallywrapped tube. The pressure vs. diameter test described abovedemonstrated that the tube had a stop point at about 14 mm. The tuberecovered to approximately its original diameter upon removal of theinternal pressure.

The fibrils of the membrane were noted to have a serpentine shape asshown in FIG. 5, a scanning electron micrograph of the surface ofhelically wrapped tube with the copolymer partially removed taken at5000×.

The invention of this application has been described above bothgenerically and with regard to specific embodiments. The invention isnot otherwise limited, except for the recitation of the claims set forthbelow.

What is claimed is:
 1. A stent graft comprising: a stent having a wall with at least one opening, an outer surface, and an inner surface; and a cover affixed to said stent, said cover comprising a composite material comprising at least one expanded fluoropolymer membrane and an elastomer, wherein said expanded fluoropolymer membrane comprises serpentine fibrils,
 2. The stent graft of claim 1, wherein each said serpentine fibril has a width of about 1.0 micron or less.
 3. The stent graft of claim 2, wherein each said serpentine fibril has a width of 0.5 micron or less.
 4. The stent graft of claim 1, wherein said cover is wrinkle-free.
 5. The stent graft of claim 1, wherein said cover is affixed to said outer surface.
 6. The stent graft of claim 1, wherein said cover is affixed to said inner surface.
 7. The stent graft of claim 1, wherein said cover is affixed to said outer surface and said inner surface.
 8. The stent graft of claim 1, wherein said elastomer is present in pores of said fluoropolymer membrane.
 9. The stent graft of claim 1, wherein said at least one expanded fluoropolymer membrane comprises a plurality of pores and said elastomer is present in substantially all of said pores.
 10. The stent graft of claim 1, wherein the fluoropolymer membrane comprises polytetrafluoroethylene.
 11. The stent graft of claim 1, wherein the fluoropolymer membrane comprises a microstructure of substantially only fibrils.
 12. The stent graft of claim 1, wherein the fluoropolymer membrane comprises a plurality of serpentine fibrils.
 13. The stent graft of claim 1, wherein the expanded fluoropolymer membrane comprises a microstructure of substantially only serpentine fibrils.
 14. The stent graft of claim 1, wherein said composite material can be radially expanded up to a diameter beyond which further expansion is inhibited.
 15. The stent graft of claim 1, wherein said cover is wrinkle free when radially expanded to about 90% of a nominal diameter.
 16. The stent graft of claim 1, wherein said cover is wrinkle free when radially expanded to about 85% of a nominal diameter.
 17. The stent graft of claim 1, wherein said cover is wrinkle-free when radially expanded to about 80% of a nominal diameter.
 18. The stent graft of claim 1, wherein said composite material is free of wrinkles before loading and after deployment of said device.
 19. The stent graft of claim 1, wherein said composite material is circumferentially wrapped around said stent.
 20. The stent graft of claim 1, wherein said composite material is axially wrapped around said stent.
 21. The stent graft of claim 1, wherein said composite material is helically wrapped around said stent.
 22. The stent graft of claim 1, wherein said composite material is both circumferentially and axially wrapped around said stent.
 23. The stent graft of claim 1, wherein the stent comprises a self-expanding stent.
 24. The stent graft of claim 1, wherein elastic properties are present in the axial direction of the stent graft.
 25. The stent graft of claim 1, wherein said elastomer is selected from the group consisting of perfluoromethylvinyl ether-tetrafluoroethylene copolymers, perfluoro(alkyl vinyl ether)-tetrafluoroethylene copolymers, silicones and polyurethanes.
 26. An implantable graft comprising: a tubular member comprising at least one expanded fluoropolymer membrane and an elastomer, wherein said expanded fluoropolymer membrane comprises serpentine fibrils having a width of about 1.0 micron or less.
 27. The implantable graft of claim 26, wherein each said serpentine fibril has a width of about 0.5 micron or less.
 28. The implantable graft of claim 26, wherein said tubular member is wrinkle-free when radially expanded to about 80%.
 29. The graft of claim 26, wherein said tubular member has elastic properties in the axial direction of said tubular member.
 30. The graft of claim 26, wherein said tubular member has elastic properties in the radial direction of said tubular member.
 31. The graft of claim 26, wherein said elastomer is present in pores of said fluoropolymer membrane.
 32. The graft of claim 26, wherein said elastomer is from the group consisting of perfluoromethylvinyl ether-tetrafluoroethylene copolymers, perfluoro(alkyl vinyl ether)-tetrafluoroethylene copolymers, silicones and polyurethanes.
 33. The graft of claim 26, wherein said fluoropolymer membrane comprises polytetrafluoroethylene.
 34. The graft of claim 26, wherein said fluoropolymer membrane comprises a plurality of pores and said elastomer is present in substantially all of the pores of said fluoropolymer membrane.
 35. The graft of claim 26, wherein the fluoropolymer membrane comprises a microstructure of substantially only fibrils.
 36. The graft of claim 26, wherein the fluoropolymer membrane comprises a plurality of serpentine fibrils.
 37. The graft of claim 26, wherein the expanded fluoropolymer membrane comprises a microstructure of substantially only serpentine fibrils.
 38. The graft of claim 26, wherein said tubular member can be radially expanded up to diameter beyond which further expansion is inhibited.
 39. A method of manufacturing a stent graft comprising: positioning a cover comprising a composite material under tension sufficient to overcome the retraction force of said composite material on at least one surface of a stent at an expanded diameter of said stent to form a stent graft; and bonding said cover to said stent, wherein said composite material comprises at least one expanded fluoropolymer membrane and an elastomer, and wherein said expanded fluoropolymer membrane comprises serpentine fibrils.
 40. The method of claim 39, wherein each said serpentine fibril has a width of about 1.0 micron or less.
 41. The method of claim 40, wherein each said serpentine fibril has a width of about 0.5 micron or less.
 42. The method of claim 39, wherein said cover is wrinkle-free before loading and after deployment.
 43. The method of claim 39, wherein said cover is positioned on at least one of an outer surface and an inner surface of said stent.
 44. The method of claim 39, wherein said at least one expanded fluoropolymer membrane comprises a plurality of pores and said elastomer is present in substantially all of said pores.
 45. The method of claim 39, further comprising applying an adhesive to said stent prior to positioning said cover on said at least one surface of said stent.
 46. The method of claim 39, wherein the expanded fluoropolymer membrane comprises a microstructure of substantially only serpentine fibrils,
 47. The method of claim 39, wherein the fluoropolymer membrane comprises polytetrafluoroethylene.
 48. The method of claim 39, wherein the fluoropolymer membrane comprises a microstructure of substantially only fibrils.
 49. The method of claim 39, wherein when said cover is wrinkle-free when expanded to about 80% of a nominal diameter.
 50. The method of claim 39, wherein said cover has a diameter substantially larger than said nominal diameter.
 51. The method of claim 39, wherein said bonding step occurs substantially at room temperature.
 52. The method of claim 39, wherein said cover in a relaxed state has a diameter substantially smaller than said expanded diameter of said stent.
 53. The method of claim 39, wherein the fluoropolymer membrane comprises a plurality of serpentine fibrils.
 54. A stent graft comprising: a stent having a wall with at least one opening, an outer surface, and an inner surface; and a cover affixed to said stent at a size greater than said stent diameter, said cover comprising a composite material comprising at least one expanded fluoropolymer membrane and an elastomer, wherein said cover is free of wrinkles when radially expanded to about 80% of said stent diameter.
 55. The stent graft of claim 54, wherein said cover is affixed to said outer surface.
 56. The stent graft of claim 54, wherein said cover is affixed to said inner surface.
 57. The stent graft of claim 54, wherein said cover is affixed to said outer surface and said inner surface.
 58. The stent graft of claim 54, wherein said elastomer is present in pores of said fluoropolymer membrane.
 59. The stent graft of claim 54, wherein the fluoropolymer membrane comprises a microstructure of substantially only fibrils.
 60. The stent graft of claim 54, wherein the fluoropolymer membrane comprises a plurality of serpentine fibrils.
 61. The stent graft of claim 54, wherein each said serpentine fibril has a width of about 1.0 micron or less.
 62. The stent graft of claim 61, wherein each said serpentine fibril has a width of about 0.5 micron or less.
 63. The stent graft of claim 54, wherein the expanded fluoropolymer membrane comprises a microstructure of substantially only serpentine fibrils.
 64. The stent graft of claim 54, wherein said composite material can be radially expanded up to a diameter beyond which further expansion is inhibited.
 65. A stent graft comprising: a stent having a wall with at least one opening, an outer surface, and an inner surface; and a cover comprising at least one expanded fluoropolymer membrane and an elastomer, said cover being provided on at least one of said outer surface and said inner surface and covering at least a portion of the at least one opening, wherein said expanded fluoropolymer membrane comprises serpentine fibrils.
 66. The stent graft of claim 65, wherein each said serpentine fibril has a width of about 1.0 micron or less.
 67. The stent graft of claim 66, wherein each said serpentine fibril has a width of about 0.5 micron or less.
 68. The stent graft of claim 65, wherein said cover is wrinkle-free when radially expanded to about 80% of a nominal diameter.
 69. The stent graft of claim 65, wherein said elastomer is present in pores of said fluoropolymer membrane.
 70. The stent graft of claim 65, wherein said expanded fluoropolymer membrane comprises a plurality of pores and said elastomer is present in substantially all of said pores.
 71. The stent graft of claim 65, wherein the fluoropolymer comprises polytetrafluoroethylene.
 72. The stent graft of claim 65, wherein the fluoropolymer membrane comprises a plurality of serpentine fibrils.
 73. The stent graft of claim 65, wherein the fluoropolymer membrane comprises a microstructure of substantially only fibrils.
 74. The stent graft of claim 65, wherein said fluoropolymer membrane is circumferentially wrapped around said stent.
 75. The stent graft of claim 65, wherein said fluoropolymer membrane is axially wrapped around said stent.
 76. The stent graft of claim 65, wherein said fluoropolymer membrane is helically wrapped around said stent.
 77. The stent graft of claim 65, wherein said fluoropolymer membrane is both circumferentially and axially wrapped around said stent.
 78. The stent graft of claim 65, wherein the stent comprises a self-expanding stent.
 79. The stent graft of claim 65, wherein the cover is provided on the inner surface of the stent.
 80. The stent graft of claim 65, wherein the cover is provided on the outer surface of the stent.
 81. The stent graft of claim 65, wherein the covering is provided on the outer and inner surfaces of the stent.
 82. The stent graft of claim 65, wherein elastic properties are present in the axial direction of the stent graft.
 83. The stent graft of claim 65, wherein elastic properties are present in the radial direction of the stent graft.
 84. The stent graft of claim 65, wherein said cover can be radially expanded up to a diameter beyond which further expansion is inhibited.
 85. The stent graft of claim 65, wherein the expanded fluoropolymer membrane comprises a microstructure of substantially only serpentine fibrils.
 86. The stent graft of claim 65, wherein said elastomer is selected from the group consisting of perfluoromethylvinyl ether-tetrafluoroethylene copolymers, perfluoro(alkyl vinyl ether)-tetrafluoroethylene copolymers, silicones and polyurethanes.
 87. A method of manufacturing a tubular member comprising: positioning an expanded fluoropolymer membrane having therein an elastomer under tension sufficient to overcome the retraction force of said fluoropolymer membrane onto a mandrel; bonding said fluoropolymer membrane to itself to form a tubular member; and removing said tubular membrane from said mandrel, wherein said expanded fluoropolymer membrane comprises serpentine fibrils.
 88. The stent graft of claim 87, wherein each said serpentine fibril has a width of about 1.0 micron or less.
 89. The stent graft of claim 88, wherein each said serpentine fibril has a width of about 0.5 micron or less. 